This invention generally relates to telephony networks and, more specifically, to a telephony switching system that is characterized by switching calls to remotely located port groups in the network.
U.S. patent application Ser. No. 924,883 discloses a telephony network that includes a digital central office for performing switching operations. In thin network, telephone lines from subscribers and trunk circuits from other central offices connect directly to a digital central office through a plurality of line and trunk circuits in port group units. The connection is made through conventional tip and ring, or analogous, conductors that extend from each individual subscriber or office to the location of the digital central office. These conductors carry signals in analog form that represent voice information and supervisory information. Supervisory information received by the central office is called "sense supervisory" information and includes hook status and dial pulse information; supervisory information sent to the port group units and the conductors is called "control supervisory" information and includes ringing and other information.
Each port group unit connects to a plurality of telephone lines through individual port circuits, such as line or trunk circuits. Each port circuit converts the incoming voice signals to pulse-code-modulated signals that are multiplexed and transferred in a serial pulse train onto a port group highway. Sense supervisory information also is multiplexed into this pulse train.
A tire slot interchange (TSI) matrix network receives this pulse train and strips the incoming sense supervisory information for storage in an area of a port data store that is assigned to each port circuit. A port event processor samples the information in each port data store area and modifies and uses the information in that area to send messages to a call control processor. The call control processor sends information including commands to the individual areas of the port data store for enabling the port event processor to control the corresponding telephone subscriber's line and to the TSI matrix network for establishing a switching channel through the network to thereby establish a path for the digitized voice signals over the same or another port group highway to a called telephone.
Commands to the port event processor from the call control processor enable the transmission of a dial tone, the termination of a dial tone, or the ringing of both the called and calling telephones. The port event processor generates control supervisory information in response to these commands. The control supervisory information is interspersed with the voice information in digital form for transmission to the port group unit connected to the port group highway. Then the corresponding port group unit performs various functions in response to the commands and converts the digital voice data signals into analog form for transmission through a particular port circuit to the subscriber's telephone lines and telephone.
U.S. patent application Ser. No. 924,883 discloses a single call processing system of this general construction that operates as a digital central office. U.S. Pat. No. 4,276,451 depicts a digital central office that includes two such call processing systems that operate in parallel and in conjunction with a maintenance processor. The two parallel call processing systems receive incoming signals from the subscribers and trunk lines simultaneously and operate in synchronism. However, signals from only one of the call processing systems pass through the port group units to the subscriber and trunk lines. The maintenance processor system analyzes losses of synchronization between the two call processing systems, parity errors, and other conditions. It determines which of the two parallel call processing systems actually controls the telephony network.
Digital central offices of this type require individual telephone lines from each subscriber location to the location of the digital central office. This approach works well and is economical where the subscribers either are located in a relatively small geographical area around the digital central office or are randomly, but widely, dispersed geographically. However, in many applications, telephony network subscribers tend to be located in geographically remote clusters. For example, subscribers may live in several small towns in a rural setting or may live in different apartment houses in an urban setting.
Telephony networks in such applications incorporate a great deal of redundant cabling to effect the individual connections. This cabling requires more than electrical conductors. Various gain devices may be required along these conductors; usually at least one gain device is required for each subscriber line. Thus, the cabling costs increase dramatically as the number of subscribers increases. Moreover, in many situations, the traffic volume in such networks, as a percentage of maximum traffic capacity, is very low. Thus, the actual utilization of the telephone lines can become quite expensive and inefficient.
Concentrators enable an efficient utilization of data channels in digital data networks. Basically, a digital data network includes modulator/demodulator ("modem") circuits for enabling digital information to be transferred over a normal telephone network in an analog form. When several subscribers in one area require only low-speed data transfers, each subscriber is connected to a local concentrator at a particular location by means of two low-speed modems; one at subscriber's location and the other at the concentrator location. The concentrator location will have one such low-speed modem for each incoming telephone line. A digital processing circuit converts the digital signals between the low-speed modems of the concentrator and a time multiplexed, high-speed, serial, digital pulse train that is applied to and received from a high-speed modem that is in a high-speed path to a data processing center. Oftentimes these concentrators are very sophisticated and an apparently large concentration can occur at such a point.
However, in many applications the actual concentration is less than 40:1. Moreover, this approach is not readily adapted for application to conventional, voice telephony. The high speed data networds require specially conditioned telephone lines that are expensive to utilize, and the required modems are expensive. The modems produce or respond to carriers held to a finite frquency band and digital processing circuits are, in effect, independent switches that can become quite complex and expensive. In addition, even if readily adapted to a telephony network, the economic benefit of substituting this type of a concentrator network at a remote location in a telephony system would not be economically justified by the cabling savings that would otherwise be provided.
Another approach that is applicable to a telephony network is to place remote port units at the center of the subscriber clusters and establish a reduced number of communications links, for example one to three links, between each of those clusters and a special unit that connects in place of one or more lines to the analog portion of the digital central office. However, it is difficult with this system to provide many of the maintenance functions that are provided in the conventional digital central office, such as disclosed in U.S. Pat. No. 4,276,451.
One such maintenance function is a test call function that is described in U.S. Pat. No. 4,276,451. In such a system, the maintenance processor and related circuitry are able to effectively disconnect two telephone lines from their respective subscribers and to complete a predetermined call from one such line to another through test access relays. This provides a very useful maintenance tool, especially for performing preventive maintenance and other diagnostic functions. In this system, however, all port group units are at the digital central office so the interconnection between the maintenance processor and related circuitry and the test access relays at the port group units and port circuits is easily achieved. There is also a single test call generator that, through the test access relays and other switching networks, provides all the test calls. However, this type of test call generator is not readily adapted to an application of remote port units. Any such application would require separate connections to the test access relays, with increased cabling costs. Moreover, various tones have to be produced and transmitted and the information necessary for controlling such tones is difficult to obtain.